Nanocomposites containing nano-scaled fillers possess unique features and functions unavailable in conventional fiber-reinforced polymers. One major filler development in the past two decades is the carbon nanotube (CNT), which has a broad range of nanotechnology applications. However, attempts to produce CNT in large quantities have been fraught with overwhelming challenges due to poor yield and costly fabrication and purification processes. Hence, even the moderately priced multi-walled CNTs remain too expensive to be used in high volume polymer composite and other functional or structural applications. Further, for many applications, processing of nanocomposites with high CNT concentrations has been difficult due to the high melt viscosity.
Instead of trying to develop lower-cost processes for CNTs, the applicants sought to develop an alternative nanoscale carbon material with comparable properties that can be produced much more cost-effectively and in larger quantities. This development work led to the discovery of processes and compositions for a new class of nano material now commonly referred to as nano graphene platelets (NGPs), graphene nano sheets, or graphene nano ribbons [e.g., B. Z. Jang and W. C. Huang, “Nano-scaled graphene plates,” U.S. Pat. No. 7,071,258, Jul. 4, 2006].
An NGP is a platelet, sheet, or ribbon composed of one or multiple layers of graphene plane, with a thickness as small as 0.34 nm (one carbon atom thick). A single-layer graphene is composed of carbon atoms forming a 2-D hexagonal lattice through strong in-plane covalent bonds. In a multi-layer NGP, several graphene planes are weakly bonded together through van der Waals forces in the thickness-direction. Multi-layer NGPs can have a thickness up to 100 nm. Conceptually, an NGP may be viewed as a flattened sheet of a carbon nano-tube (CNT), with a single-layer graphene corresponding to a single-wall CNT and a multi-layer graphene corresponding to a multi-wall CNT. However, this very difference in geometry also makes electronic structure and related physical and chemical properties very different between NGP and CNT. It is now commonly recognized in the field of nanotechnology that NGP and CNT are two different and distinct classes of materials.
NGPs are predicted to have a range of unusual physical, chemical, and mechanical properties and several unique properties have been observed. For instance, single-layer graphene (also referred to as single-sheet NGP) was found to exhibit the highest intrinsic strength and highest thermal conductivity of all existing materials [C. Lee, et al., “Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene,” Science, 321 (July 2008) 385-388; A. Balandin, et al. “Superior Thermal Conductivity of Single-Layer Graphene,” Nano Lett., 8 (3) (2008) 902-907]. Single-sheet NGPs possess twice the specific surface areas compared with single-walled CNTs. In addition to single-layer graphene, multiple-layer graphene platelets also exhibit unique and useful behaviors. Single-layer and multiple-layer graphene are herein collectively referred to as NGPs. Graphene platelets may be oxidized to various extents during their preparation, resulting in graphite oxide or graphene oxide (GO) platelets. In the present context, NGPs refer to both “pristine graphene” containing no oxygen and “GO nano platelets” of various oxygen contents. It is helpful to herein describe how NGPs are produced.
The processes that have been used to prepare NGPs were recently reviewed by the applicants [Bor Z. Jang and A Zhamu, “Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: A Review,” J. Materials Sci. 43 (2008) 5092-5101]. As illustrated in FIG. 1, the most commonly used process entails treating a natural graphite powder (referred to as Product (A) in FIG. 1) with an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or, actually, graphite oxide (GO) (referred to as Product (B) in FIG. 1). Prior to intercalation or oxidation, graphite has an inter-graphene plane spacing of approximately 0.335 nm (Ld=½d002=0.335 nm or 3.35 Å, based on X-ray diffraction data readily available in open literature). There is a misconception in the scientific community that van der Waals forces are weak forces, which needs some qualifications. It is well-known that van der Waals forces are short range forces, but can be extremely strong in magnitude if the separation between two objects (e.g., two atoms or molecules) is very small, say <0.4 nm. However, the magnitude of van der Waals forces drops precipitously when the separation increases just slightly. Since the inter-graphene plane distance in un-intercalated and un-oxidized graphite crystal is small (<0.34 nm), the inter-graphene bonds (van der Waals forces) are actually very strong.
With an intercalation or oxidation treatment, the inter-graphene spacing is increased to a value typically greater than 0.55-0.65 nm. This is the first expansion stage experienced by the graphite material. The van der Waals forces are now significantly weakened due to the increased spacing. It is important to note that, in most cases, some of the graphene layers in a GIC are intercalated (with inter-graphene spacing increased to 0.55-0.65 nm and van der Waals forces weakened), but other layers could remain un-intercalated or incompletely intercalated (with inter-graphene spacing remaining approximately 0.34 nm and van der Waals forces staying strong).
In the conventional processes, the obtained GIC or GO, dispersed in the intercalant solution, will need to be rinsed for several cycles and then dried to obtain GIC or GO powders. These dried powders, commonly referred to as expandable graphite, are then subjected to further expansion or second expansion (often referred to as exfoliation) typically using a thermal shock exposure approach (at a temperature from 650° C. to 1,100° C.). The acid molecules residing in the inter-graphene spacing are decomposed at such a high temperature, generating volatile gas molecules that could push apart graphene planes. The inter-flake distance between two loosely connected flakes or platelets is now increased to the range of typically >20 nm to several μm (hence, very weak van der Waals forces).
Unfortunately, typically a significant portion of the gaseous molecules escape without contributing to exfoliation of graphite flakes. Further, those un-intercalated and incompletely intercalated graphite layers remain intact (still having an inter-graphene spacing of approximately <0.34 nm). Additionally, many of the exfoliated flakes re-stack together by re-forming van der Waals forces if they could not be properly separated in time. These effects during this exfoliation step led to the formation of exfoliated graphite (referred to as Product (C) in FIG. 1), which is commonly referred to as “graphite worm” in the industry.
The exfoliated graphite or graphite worm is characterized by having networks of interconnected (un-separated) flakes which are typically >50 nm thick (often >100 nm thick). These individual flakes are each composed of hundreds of layers with inter-layer spacing of approximately 0.34 nm (not 0.6 nm), as evidenced by the X-ray diffraction data readily available in the open literature. In other words, these flakes, if separated, are individual graphite particles, rather than graphite intercalation compound (GIC) particles. This thermal shock procedure can produce some separated graphite flakes or graphene sheets, but normally the majority of graphite flakes remain interconnected. Again, the inter-flake distance between two loosely connected flakes or platelets is now increased to from 20 nm to several μm and, hence, the ven der Waals forces that hold them together are now very weak, enabling easy separation by mechanical shearing or ultrasonication.
Typically, the exfoliated graphite or graphite worm is then subjected to a sheet or flake separation treatment using air milling, mechanical shearing, or ultrasonication in a liquid (e.g., water). Hence, a conventional process basically entails three distinct procedures: first expansion (oxidation or intercalation), further expansion (so called “exfoliation”), and separation. The resulting NGPs are graphene oxide (GO), rather than pristine graphene.
It is important to note that the separation treatment (e.g. using ultrasonication or shearing) is to separate those thick flakes from one another (breaking up the graphite worm or sever those weak interconnections), and it is not intended for further peeling off individual graphene planes. In the prior art, a person of ordinary skill would believe that ultrasonication is incapable of peeling off non-intercalated/un-oxidized graphene layers. In other words, in the conventional processes, the post-exfoliation ultrasonication procedure was meant to break up graphite worms (i.e., to separate those already largely expanded/exfoliated flakes that are only loosely connected). Specifically, it is important to further emphasize the fact that, in the prior art processes, ultrasonification is used after intercalation and oxidation of graphite (i.e., after first expansion) and most typically after thermal shock exposure of the resulting GIC or GO (i.e., after second expansion or exfoliation) to aid in breaking up those graphite worms. There are already much larger spacings between flakes after intercalation and/or after exfoliation (hence, making it possible to easily separate flakes by ultrasonic waves). This ultrasonication was not perceived to be capable of separating those un-intercalated/un-oxidized layers where the inter-graphene spacing remains <0.34 nm and the van der Waals forces remain strong.
To the best of our knowledge, the applicant's research group was the very first in the world to surprisingly observe that, under proper conditions (e.g., with the assistance of a surfactant), ultrasonication can be used to produce ultra-thin graphene directly from graphite, without having to go through chemical intercalation or oxidation. This invention was reported in a patent application [A. Zhamu, J. Shi, J. Guo, and Bor Z. Jang, “Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano Graphene Plates,” U.S. patent Ser. No. 11/800,728 (May 8, 2007)]. Schematically shown in FIG. 2 are the essential procedures used to produce single-layer or few-layer graphene using this direct ultrasonication process. This innovative process involves simply dispersing pristine graphite powder particles in a liquid medium (e.g., water, alcohol, or acetone) containing a dispersing agent or surfactant to obtain a suspension. The suspension is then subjected to an ultrasonication treatment, typically at a temperature between 0° C. and 100° C. for 10-120 minutes. No chemical intercalation or oxidation is required. The graphite material has never been exposed to any obnoxious chemical. This process combines expansion, exfoliation, and separation into one step. Hence, this simple yet elegant method obviates the need to expose graphite to a high-temperature, or chemical oxidizing environment. The resulting NGPs are essentially pristine graphene, which is highly conductive both electrically and thermally.
This direct ultrasonication process may be considered as peeling off graphene layers at a rate of 20,000 attempts per second (if the ultrasonic frequency is 20 kHz) or higher (if higher frequency) per each suspended graphite particle. The resulting NGPs are pristine graphene without any intentionally added or bonded oxygen. This is a powerful approach to the large-scale preparation of pristine NGPs.
After additional research and development work, we have further discovered that a surfactant is not needed if the graphite particles are mixed with a certain liquid or solvent that meets a specific surface energy requirement. The resulting surfactant-free mixture of pristine graphitic particles (non-preintercalated, un-oxidized, un-fluorinated, etc) and solvent is then subjected to direct ultrasonication. This improvement is significant since it eliminates the need to remove a surfactant from this liquid or solvent. In certain applications, the surfactant removal procedure could be challenging, tedious, or expensive, particularly where the NGP-liquid suspension is intended for use directly as a product (e.g., as a conductive ink, coating, or paint) and the presence of a surfactant is undesirable.
The presently invented process is fast and environmentally benign. It can be easily scaled-up for mass production of highly conducting graphene. Again, it is important to emphasize that, in all prior art processes, ultrasonification was used after intercalation and oxidation of graphite (i.e., after first expansion) and, in most cases, after thermal shock exposure of the resulting GIC or GO (after second expansion). In contrast, the presently invented process does not involve pre-oxidizing or pre-intercalating the starting graphite particles.
It may be noted that a few other approaches to producing graphene materials also involve the use of pristine graphite as starting material. The fundamental value of such an approach lies in its avoidance of graphite oxidation and subsequent reduction (typically incomplete reduction only), thereby preserving the desirable electronic properties of graphene. There is precedent for exfoliation of pristine graphite in neat organic solvents without oxidation or surfactants. Hernandez et al reported dispersion of natural graphite using N-methylpyrrolidone (NMP), resulting in individual sheets of graphene at a concentration of ≦0.01 mg/mL [Y. Hernandez, et al, “High-yield production of graphene by liquid phase exfoliation of graphite,” Nature Nanotechnology, 2008, 3, 563]. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene, resulting in a minimal enthalpy of mixing and possibly negative Gibbs free energy of mixing. This is a liquid phase “dissolution” approach. NMP is a highly polar solvent, and not ideal in cases where reaction chemistry requires a nonpolar medium. Further, it is hygroscopic, making its use problematic when water must be excluded from reaction mixtures.
Hence, it was an object of the present invention to provide a method of mass-producing a pristine nano graphene material that has good electrical conductivity.
It was another object of the present invention to provide a process for mass-producing pristine NGPs without involving the use of any undesirable acid, oxidizing agent, chemical reducing agent, or a surfactant.
It was a further object of the present invention to provide a process for producing ultra-thin NGPs (e.g., those with a thickness less than 1 nm).
Another object of the present invention was to provide a process capable of directly producing a dispersion product containing pristine NGPs and a liquid medium, without having to remove a surfactant or any other chemical from the dispersion.
Still another object of the present invention was to provide a process capable of directly producing a precursor to a composite product, wherein the precursor contains pristine NGPs dispersed in a surfactant-free liquid medium (e.g., a solvent) and a monomer or polymer dissolved therein. This process does not require removal of a surfactant or any other chemical from the solvent. When the solvent is vaporized and monomer cured or synthesized, an NGP-reinforced polymer composite is produced.
A further object of the invention was to provide a versatile process for mass-producing pristine NGPs from a broad range of pristine graphitic materials (e.g. natural graphite, artificial graphite, MCMBs, carbon or graphite fibers, carbon or graphite nano-fibers, graphitic cokes, meso-phase carbon, soft carbon, hard carbon, graphitized pitch, and combinations thereof).